Structure and method of making field effect transistor having multiple conduction states

ABSTRACT

A field effect transistor (“FET”) is provided has a semiconductor region including a channel region, a source region and a drain region and a gate conductor overlying the channel region. Such FET has a first threshold voltage having a first magnitude and a second threshold voltage having a second magnitude higher than the first magnitude, both threshold voltages being effective at the same time. The FET is operable in response to a gate-source voltage between the gate conductor and the source region in multiple states including at least: a) an essentially nonconductive state when a magnitude of the gate-source voltage is less than the first magnitude and less than the second magnitude, the source-drain current then being at most a negligible value; b) a first conductive state when the magnitude of the gate-source voltage is greater than the first magnitude and less than the second magnitude, the source-drain current then having a first operating value about ten or more times higher than the negligible value; and c) a second conductive state when the magnitude of the gate-source voltage is greater than first magnitude and the second magnitude, in which state the source-drain current has a second operating value ten or more times higher than the first operating value.

BACKGROUND OF THE INVENTION

The present invention relates to transistor devices, and more particularly to a field effect transistor device having multiple conduction states.

Transistors are a basic building block of both digital and analog circuits. In analog circuits, transistors are frequently used as linear amplifiers in which a transistor is biased to remain always “on” in a linear region of operation. On the other hand, in digital circuits, transistors are most commonly used as on-off switches. However, for certain applications, a circuit function is needed which cannot be neatly classified as either linear amplification or on-off switching. For such applications, multiple transistors can be arranged in a circuit together to accomplish a required function. However, circuits which utilize a greater number of transistors tend to occupy a larger area of the chip than those circuits which utilize fewer transistors. Moreover, on an integrated circuit or “chip”, circuits which include multiple transistors can be replicated many thousands or millions of times. The decision to implement a circuit function requiring a greater number of transistors must be weighed against its affect on the area utilization of the whole chip, as it may force additional decisions to reduce the area occupied by other circuitry on the chip.

U.S. Pat. No. 5,741,738 to Mandelman et al. describes a problem of “corner” conduction in field effect transistors (“FETs”) formed in semiconductor regions bounded by shallow trench isolations. Along the vertically oriented edges of the shallow trench isolations at the “corners” of the device, an uncorrected FET can exhibit higher than usual leakage current when the transistor is biased in an “off” state, due to unwanted low-level conduction at the corners. By definition, the corner conduction is an uncontrolled effect along vertical edges of a transistor device, in contrast to the normal controlled conduction which occurs substantially in the direction of a plane just below a gate dielectric at the major surface of the transistor. The corner conduction serves no useful purpose. One focus of the prior art has been to correct for or eliminate the effects of corner conduction, since it is an unwanted, uncontrolled parasitic effect.

SUMMARY OF THE INVENTION

According to one aspect of the invention, a field effect transistor (“FET”) is provided which includes a semiconductor region in which a channel region, a source region and a drain region are provided. A gate conductor overlies the channel region, wherein the FET has a first threshold voltage having a first magnitude and a second threshold voltage having a second magnitude higher than the first magnitude. The first threshold voltage and the second threshold voltage are effective at the same time, in that the FET is operable in response to a gate-source voltage between the gate conductor and the source region in multiple states including: a) an

essentially nonconductive state when a magnitude of the gate-source voltage is less than the first magnitude and less than the second magnitude such that a source-drain current between the source region and the drain region has a negligible value; b) a

first conductive state when the magnitude of the gate-source voltage is greater than the first magnitude and less than the second magnitude in which state the source-drain current has a first operating value of about ten or more times higher than the negligible value; and c)

a second conductive state when the magnitude of the gate-source voltage is greater than first magnitude and the second magnitude and in which state the source-drain current has a second operating value of about ten or more times higher than the first operating value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-3 illustrate a structure of a multiple conductive state FET in accordance with an embodiment of the invention and a method of making the same.

FIG. 4 is a graph illustrating operation of a multiple conductive state FET in accordance with an embodiment of the invention.

FIGS. 5-6 illustrate a structure of a multiple conductive state FET in accordance with another embodiment of the invention and a method of making the same.

FIGS. 7-9 illustrate a structure of a multiple conductive state FET in accordance with yet another embodiment of the invention.

DETAILED DESCRIPTION

In accordance with an embodiment of the invention, a new type of transistor is provided which can be used to perform a circuit function which previously required multiple transistors to implement. The novel transistor device, specifically, a field effect transistor (“FET”) device, is provided which has at least three discrete operational states instead of the two “on” and “off” states of conventional transistors. The three discrete states include: a) an essentially nonconductive state in which the FET conducts little or no current; b) a “low-conductive” or “off” state in which the FET is operated by a gate-source voltage exceeding a first threshold voltage such that the FET conducts, but with reduced current; and c) a “high-conductive” state in which the FET is operated by the gate-source voltage exceeding a second threshold voltage as well as the first threshold voltage such that the FET conducts significantly greater amount of current than the reduced current amount. In like manner, the FET can have additional discrete operational states corresponding to the gate bias condition in relation to a third threshold voltage, a fourth threshold voltage, and so on. The first and second threshold voltages are effective at the same time, because the first and second threshold voltages are determined by the structure that simultaneously exists in different parts of the transistor. Thus, the FET moves between the off, the low conductive state and the high conductive state without needing to be “programmed” or “reprogrammed” in the interim.

Referring to FIG. 1, a first preferred embodiment of the invention will now be described. FIG. 1 is a top-down plan view illustrating a device structure of an FET 1 00 in accordance with one embodiment of the invention. As shown in FIG. 1, FET 100 is disposed in a semiconductor region 102 of a substrate, e.g., a wafer, the semiconductor region 102 being isolated via an isolation structure 104 such as shallow trench isolation (“STI”). The FET can either be an n-type field effect transistor (“NFET”) or a p-type field effect transistor (“PFET”). A source region 106 and a drain region 108 are disposed within the semiconductor region 102, these regions occupying portions of the semiconductor region 102 separated from each other by a gate conductor 110. The active area of the FET depicted in FIG. 1 has two transistor portions, delineated by dotted line 112. The two portions of the active area extend in an at least generally planar direction at the upper surface (140; FIG. 2) of the semiconductor region 102.

The two portions operate according to different threshold voltages. A first portion 114 has a structure that results in such portion turning on upon exceeding a first threshold voltage, conducting a first operating current when turned on. The second portion 116 has a structure that results in such portion turning on upon exceeding a second threshold voltage, a voltage which is higher than the first threshold voltage. The second portion 116 also conducts a second operating current when it is turned on that is higher than the first operating current value. The value of the first and second threshold voltages and the amount of current to be conducted in each conductive state are selected by design a prioriand are set by the particular structure of the transistor. Illustratively, the second threshold voltage has a magnitude which is about 100 mV to 200 mV higher than the magnitude of the first threshold voltage. Illustratively, the amount of the second operating current is significantly greater than the amount of the first operating current, e.g., one to two orders of magnitude greater. Preferably, the difference between the magnitude of the second operating current and that of the first operating current is not as high as the difference in current, e.g., about 1000 times, between that of a fully turned “on” transistor and the subthreshold leakage current of a transistor, i.e., one that is considered to be in the “off” state.

The first and second portions of the FET represent a functional division of the FET in a direction of the width of the FET. In one preferred embodiment, the width 122 of the channel region within the first portion 114 is about equal to the width 124 of the channel region within the second portion 116. However, in other embodiments described below, the difference between these widths 122,124 varies.

Turning now to FIG. 2, a corresponding sectional view of FET 100 through lines 2-2 of FIG. 1 is now shown. As depicted in FIG. 2, the semiconductor region 102 is bounded by an STI region 104. As also shown therein, the gate conductor 110 overlies the semiconductor region 102 and is separated therefrom by a gate dielectric. In the embodiment shown in FIG. 2, the gate dielectric overlying both the first portion 114 and the second portion 116 is at least substantially co-planar. The gate dielectric has a first portion 130 having a first nominal thickness and a second portion 132 having a second nominal thickness, the second nominal thickness being substantially thinner than the first nominal thickness. In a particular example, the thin gate dielectric utilized as the second portion 132 has thickness of about 10 angstroms and the thicker gate dielectric used in the first in the first portion has a thickness between about 15 angstroms and 50 angstroms, being about 1.5 to 5 times thicker than the gate dielectric of the second portion. Preferably, the second nominal thickness is about 1.5 to about five times the first nominal thickness. The thicknesses of each portion of the gate dielectric are preferably controlled independently in accordance with the process embodiments described below.

In one embodiment, the boundary between the first and second portions marks a step difference in thickness. The first portion 130 of the gate dielectric extends across the first portion 114 of the transistor, while the second portion 132 of the gate dielectric extends across the second portion 116 of the transistor. Dopant concentrations near a top surface 140 of the semiconductor region 102 preferably vary between the first portion 114 and the second portion 116 of the FET. These dopant concentrations are those which are normally established through a process known as a “threshold voltage adjustment implant” process. Such process is conducted here, for example, by protecting the portion of the transistor to receive the lower dose by a patterned mask, e.g., a photoresist layer, when the higher dose is implanted. After this process, in one example, the resulting dopant concentration near the top surface 140 of the second portion has a value significantly higher, e.g., between about 1.5 times and about 100 times higher than the dopant concentration near the top surface of the first portion. In a particular example, the dopant concentration near the top surface 140 of the first portion 114 has a value of about 1×10¹⁷cm⁻³ and the dopant concentration near the top surface 140 of the second portion 116 has a value of about 7×10¹⁸cm⁻³.

By virtue of such structure, the FET 100 behaves in a manner similar to a first FET and a second FET having their gate terminals, drain terminals and source terminals conductively tied together for operation in parallel, in which the first FET has a relatively low threshold voltage and low on-current, and the second FET has a higher threshold voltage and higher on-current than that of the first FET. Specifically, a first portion 114 of the transistor 100, having the thicker gate dielectric and lower dopant concentration, operates in a manner similar to a separate transistor which has a first threshold voltage, the first portion turning on and conducting a current within the first portion of the transistor when the voltage between the source and gate of the transistor exceeds the first threshold voltage. On the other hand, the second portion 116 of the transistor, having the thinner gate dielectric and higher dopant concentration, operates in a manner similar to a separate transistor which has a second threshold voltage having a higher magnitude than the first threshold voltage, the second portion turning on and conducting a current within the second portion of the transistor only when the voltage between the gate and the source of the transistor exceeds the second threshold voltage.

The value of the second threshold voltage corresponding to the second portion of the transistor 100 is higher than the first threshold voltage because of the difference in the thicknesses of the gate dielectric layer overlying each portion, as well as the difference in the concentrations of a dopant used to perform the voltage threshold adjustment implant in the first portion 114 of the transistor relative to the second portion 116. This leads to a result which is not known to the inventors to exist in any transistor according to the prior art: namely, a single field effect transistor which has multiple discrete threshold voltages such that the FET is operable in multiple discrete conduction states having predetermined discrete output current levels. Hence, the FET has at least three discrete operational states including an essentially nonconductive state, a first conductive state, and a second conductive state. In the first conductive state, the gate-source voltage does not exceed either the first threshold voltage or the second threshold voltage, such that the FET is essentially nonconductive. In this state, the current between the source and the drain of the FET is negligible. As used herein, a “negligible” current is a current which is below a level which meaningfully affects function of the circuit which includes the FET. In this state, while some current may be conducted between the source and the drain of the FET as a “leakage current,” the FET is considered to be “off” for the purposes to the FET is utilized within a circuit.

In the first conductive state, the gate-source voltage exceeds the first threshold voltage but does not exceed the second threshold voltage. In this state, the transistor operates in a “low conduction” mode, since only the first portion of the transistor having the first threshold voltage is turned on, whereas the second portion of the transistor has not yet turned on at the time. In this low conduction mode, since only the first portion of FET is biased above its threshold voltage, the transistor conducts only a relatively low amount of current. This is true whether the FET is biased with a relatively low voltage between the drain and the source, a moderate voltage, or is biased in saturation. The range of the current which is conducted for different values of the drain-source voltage up to a saturation value remains relatively low. Typically, the value of the current at this time is about 100 or more times greater than the “leakage current” which is conducted when the transistor is nominally in the “off” state as described above.

On the other hand, the second conductive state is a “high” conduction mode in which the gate-source voltage exceeds both the first threshold voltage and the second threshold voltage. As a result, both the first portion and the second portion of the transistor are now turned on, such that the transistor is fully conductive. In this high conduction mode, because both the first portion and the second portion of the FET are biased above their respective threshold voltages, the transistor conducts a relatively high amount of current. Consequently, a relatively high amount of current is conducted by the FET, whether the FET is biased with a relatively low voltage between the drain and the source or is biased in saturation. In this state, the current ranges between relatively high values for different values of the drain-source voltage up to a saturation value, such values ranging from about one to several orders of magnitude higher than the values of the current in the low conduction mode.

According to a particular embodiment of the invention, the first and second portions 130, 132 of the gate dielectric are fabricated by thermal oxidation of the semiconductor material present at the top surface, i.e., the “major surface” 140 of the semiconductor region 102. Thermal oxidation is performed by heating the substrate while exposing the major surface 140 of the semiconductor region 102 to an oxygen-containing ambient such as an atmosphere containing gaseous oxygen, oxygen ions, or a compound of oxygen such as steam or other compound of oxygen. When the semiconductor region 102 consists essentially of a semiconductor material such as silicon, the material of the resulting gate dielectric becomes an oxide of silicon. More preferably, a majority percentage of the oxide of silicon formed thereby is silicon dioxide, a material considered to have good properties as a gate dielectric, more preferably, it is a high majority percentage, and most preferably substantially all of the oxide formed by this process is a dioxide of silicon. In an alternative embodiment, instead of an oxidation, a thermal nitridation is performed via a similar process to form a layer preferably having a substantial or high percentage of stoichometric silicon nitride overlying the semiconductor region as a gate dielectric.

However, particular processing must be conducted in order to form the different thicknesses of the gate dielectric on the respective portions of the semiconductor region. In one embodiment, in which a thermal oxide is formed as described above over the semiconductor region 102, a masked implant is performed into the top surface 140 of the second portion 116 of the FET to implant an oxidation retardant therein. In the preferred embodiment in which the semiconductor region consists essentially of silicon, a masked implant is performed to implant nitrogen ions only the top surface 140 of the second portion 116 of the FET. At that time, the top surface 140 of the first portion 114 of the semiconductor region is masked by a masking layer, e.g., a patterned photoresist layer, so that first portion 114 does not receive the implant. After the ion implanting step, the masking layer is removed and the thermal oxidation is performed in the above-described manner.

With the nitrogen present in the second portion 116, the semiconductor material present at the top surface 140 in the second portion does not oxidize as rapidly as the semiconductor material present in the first portion of the FET. As a result, the thickness of the first portion 130 of the oxide gate dielectric layer which overlies the first portion 114 becomes greater than the thickness of the portion 132 of the oxide gate dielectric layer which overlies the second portion 116.

Alternatively, a masked oxidation process can be used to form the oxide gate dielectric layers 130, 132 of different thicknesses without implanting an oxidation retardant such as nitrogen. In such embodiment, the top surface 140 of the semiconductor region is oxidized by a first oxidation followed by a second oxidation. Illustratively, this process can be performed in one of two exemplary ways. In the first example, the first portion 114 of the semiconductor region is masked by an appropriate masking layer, and the first oxidation is conducted to form the thick oxide gate dielectric layer 130 overlying the first portion 114 of the semiconductor region. Thereafter, the masking layer is removed and the second oxidation is performed to form the thinner oxide 132 overlying the second portion 116 of the semiconductor region. During the second oxidation, some or all of the first portion 116 may be oxidized further, increasing the thickness of the oxide gate dielectric layer 130 overlying the first portion, since the first portion 114 is not protected by a masking layer at the time. Alternatively, in a second example, without first applying a masking layer the first oxidation is conducted to form a thin oxide gate dielectric layer 132 overlying both the first portion 114 and the second portion 116 of the semiconductor region. Thereafter, a masking layer is applied to cover the second portion 116 of the semiconductor region and then the second oxidation is performed to form the thicker oxide gate dielectric layer 130 which overlies the first portion 114 of the semiconductor region. Finally, the masking layer is removed. Various combinations and permutations of these processes using a masked oxidation to form the two portions 130, 132 of the oxide gate dielectric layer are also possible.

In yet another alternative process, a masked thermal nitridation process is used to form portions 130, 132 of a nitride gate dielectric layer having different thicknesses, instead of using thermal oxidation. When the semiconductor region 102 consists essentially of silicon, the resulting gate dielectric layer consists essentially of silicon nitride. This alternative process is performed in a manner similar to any of the above-described masked thermal oxidation processes except that in this process a source of gaseous nitrogen, nitrogen atoms, or nitrogen-containing molecules is supplied to the top surface 140 of the semiconductor region 102 during the thermal nitridations.

FIG. 3 illustrates a transistor 200 according to a variation of the above embodiment in which the gate dielectric is deposited rather than formed by thermal processing of the material at the surface of the semiconductor region. In the variation shown in FIG. 3, the different thicknesses of the different portions of the gate dielectric are achieved through deposition. Namely, a first portion 142 of the deposited gate dielectric has a first thickness and a second portion 144 of the gate dielectric has a second thickness substantially thinner than the first thickness. As a deposited gate dielectric, the choices of materials available therefor are much greater than that available for use in the above-described thermal process embodiment. Thus, the gate dielectric can include an oxide of a material other than a semiconductor or can include a nitride of a material other than a semiconductor. For example, the gate dielectric can include any one or more of many well-known dielectric materials having a high dielectric constant, i.e., having a high permittivity in relation to the permittivity of silicon dioxide. Examples of such materials include, but are not limited to hafnium oxide, hafnium silicates, zirconium oxide, perovskite materials, ferroelectric dielectric materials, zeolites, lead zirconium titanate or “PZT” and all other well-known high-dielectric constant materials.

Of course, the gate dielectric need not contain a high dielectric constant material. Instead, the gate dielectric can include a deposited oxide of a semiconductor or a deposited nitride of a semiconductor. In another alternative, the gate dielectric may include both an oxide of silicon and an oxide of nitride, and may have a structure such as an “O—N—O” structure having a layered structure of silicon oxide, silicon nitride and silicon oxide in order.

As further shown in the variation shown in FIG. 3, the active semiconductor region of the transistor 200 is provided as a semiconductor-on-insulator (“SOI”) layer 202 of a substrate 210 which includes a buried dielectric layer 212 which isolates the SOI layer from a bulk region 214 of the substrate 210. In a specific embodiment, the SOI layer and the bulk region consist essentially of silicon and the buried dielectric layer consists essentially of an oxide of silicon, preferably being at least mostly silicon dioxide.

FIG. 4 is a graph illustrating an example of operation of the FET shown in the embodiment described above with respect to FIGS. 1 and 2 or FIG. 3. Specifically, FIG. 4 graphs a current versus voltage characteristic of the FET. Current is graphed on a logarithmic scale in relation to the gate to source voltage (|Vgs↑) which is graphed linearly. Vgs is graphed in terms of its magnitude for ease of reference because the curves are representative of operation whether the FET is an NFET or a PFET. Three curves are shown in FIG. 4. A first curve 150 represents the current versus voltage characteristic for the first portion of the transistor having the thicker gate dielectric and the lower dopant concentration. A second curve 152 represents the current versus voltage characteristic for the second portion of the transistor having the thinner gate dielectric and the higher dopant concentration. A third curve 154 represents the current versus voltage characteristic for the entire transistor, being the sum of the current voltage characteristics for the first and second portions of the transistor combined.

The first threshold voltage of the FET occurs at approximately 0.2 volts in that for values of the gate to source voltage (|Vgs|) which exceed that level, the first portion of the transistor turns on but conducts a relatively small amount of current. Under that biasing condition, the second portion of the transistor remains off. Under this condition, the FET conducts a current of about 10 microamperes (μA). This operational condition continues until |Vgs| exceeds the second threshold voltage at about 0.4 to 0.5 volts. When the second threshold voltage is exceeded, the second portion of the transistor turns on, and conducts a current which is substantially greater than the current that the first portion conducts. Thus, the current conducted by the second portion of the transistor becomes the dominant current when |Vgs| exceeds the second threshold voltage, that current being about ten or more times larger than the current already being conducted by the first portion of the transistor.

FIGS. 5 and 6 illustrate a variation of the above-described embodiment. In the embodiments shown in FIGS. 1-2 or 3, each of the first and second portions of the transistor extends over about half the width of the transistor channel. However, there is no requirement that these widths be the same. In order to tailor the operating current that is obtained when the FET is operated in each of the low and high conductive states, the width of either the first portion or the second portion of the channel region can be made larger than that of the other portion. Referring to the top-down plan view of FIG. 5, in one example, the first portion can occupy an area of the semiconductor region selected between about 10% and about 80% of the total area of the semiconductor region at the major surface, the major surface defining a substantially planar top surface over which the gate dielectric is disposed. In the particular example shown, the first portion 316 of the transistor extends over about 30% of the channel width and the second portion 314 extends over the other 70% of the channel width. As shown in the corresponding sectional view of FIG. 6 through lines 6-6 of FIG. 5, the thicker portion 332 of the gate dielectric extends only over about 30% of the transistor channel width, while the thinner portion 330 of the gate dielectric extends about 70% of the transistor channel width. Similar to the embodiment described above with reference to FIGS. 1-3, the dopant concentration in each of the first and second portions varies correspondingly.

FIG. 7 is a top-down plan view illustrating a variation of the above embodiment in which the semiconductor region of a field effect transistor 400 includes three portions 414, 416 and 418 having correspondingly varied dopant concentrations similar to that described above. As shown in the corresponding sectional view of FIG. 8 through line 8-8 of FIG. 7, each of the three portions of the transistor has a corresponding portion 430, 432 and 434 of the gate dielectric, each such portion of the gate dielectric having a different thickness from each other portion. FIG. 8 illustrates an embodiment in which the gate dielectric is formed by a thermal process of oxidation and/or nitridation such that some of the material at the top surface 440 of the semiconductor region 402 is consumed, in a manner similar to that shown in FIG. 2. Alternatively, as shown in FIG. 9, each of the three portions 450, 452 and 454 of the gate dielectric, each having a different thickness, can be formed by deposition, in a manner similar to that described above with reference to FIG. 3.

Referring to FIG. 7 again, each of the portions 414, 416, and 418 of the transistor turns on at a different threshold voltage such that transistor 400 has a lowest, a middle, and a highest threshold voltage. The transistor 400 has three discrete conduction states determined by whether the voltage applied between the gate and the source exceeds only the lowest threshold voltage of the transistor, exceeds the middle threshold voltage, or exceeds the highest threshold voltage. In each case, the conduction states are discrete and the threshold voltages mark discrete boundaries of operation in that a significant difference in the amount of current conducted by the transistor occurs upon the gate-source voltage |Vgs| exceeding each of the lowest, middle and highest threshold voltages. Note that these conduction states of the transistor are in addition to the one “off” state it has when the voltage applied between the gate and the source does not exceed any of the threshold voltages of the transistor. By extension, according to the principles described herein, in another embodiment the transistor can be fabricated to have an even greater number of discrete portions, for example five, the transistor having a corresponding number of thicknesses of the gate dielectric and correspondingly varied dopant concentrations in each portion, for operating in a correspondingly greater number of discrete conduction states.

While the invention has been described in accordance with certain preferred embodiments thereof, those skilled in the art will understand the many modifications and enhancements which can be made thereto without departing from the true scope and spirit of the invention, which is limited only by the claims appended below. 

1. A field effect transistor (“FET”), comprising: a semiconductor region including a channel region, a source region and a drain region; and a gate conductor overlying said channel region, wherein said FET has a first threshold voltage having a first magnitude and a second threshold voltage having a second magnitude higher than said first magnitude, said first threshold voltage and said second threshold voltage being effective at the same time, said FET being operable in response to a gate-source voltage between said gate conductor and said source region in multiple states including at least: a) an essentially nonconductive state when a magnitude of said gate-source voltage is less than said first magnitude and less than said second magnitude such that a source-drain current between said source region and said drain region has a negligible value; b) a first conductive state when said magnitude of said gate-source voltage is greater than said first magnitude and less than said second magnitude in which state said source-drain current has a first operating value of about ten or more times higher than said negligible value; and c) a second conductive state when said magnitude of said gate-source voltage is greater than first magnitude and said second magnitude and in which state said source-drain current has a second operating value of about ten or more times higher than said first operating value.
 2. An integrated circuit including an FET as claimed in claim
 1. 3. The FET as claimed in claim 1 wherein said semiconductor region is disposed in a semiconductor-on-insulator (“SOI”) layer of a semiconductor-on-insulator (“SOI”) substrate, said SOI layer separated from said SOI substrate by a buried insulator layer of said SOI substrate.
 4. The FET as claimed in claim 1, further comprising a gate dielectric disposed between said channel region and said gate conductor, said gate dielectric including a first portion having a first thickness and a second portion having a second thickness substantially thinner than said first thickness, wherein said first thickness at least partly determines said first threshold voltage and said second thickness at least partly determines said second threshold voltage.
 5. The FET as claimed in claim 1, wherein said second thickness is between about one and one half and about five times thicker than said first thickness.
 6. The FET as claimed in claim 4, wherein said channel region includes a first portion having a first dopant concentration and a second portion having a second dopant concentration substantially different from said first dopant concentration, said first dopant concentration at least partly determining said first threshold voltage and said second dopant concentration at least partly determining said second threshold voltage.
 7. The FET as claimed in claim 6, wherein said second dopant concentration is greater than about one and one half times said first dopant concentration.
 8. The FET as claimed in claim 7, wherein said channel region includes a first portion having a first dopant concentration and a second portion having a second dopant concentration substantially different from said first dopant concentration, wherein said first portion of said gate dielectric is at least partially aligned to said first portion of said channel region, said second portion of said gate dielectric is at least partially aligned to said second portion of said channel region, said first dopant concentration at least partly determining said first threshold voltage and said second dopant concentration at least partly determining said second threshold voltage.
 9. A field effect transistor (“FET”), comprising: a semiconductor region including a channel region, a source region and a drain region, said source region having a conduction path through said channel region to said drain region, said channel region having a major surface including a first portion and a second portion, said first and second portions each occupying a substantial proportion of a total area of said major surface; a gate dielectric having a first portion overlying said first portion of said major surface and having a second portion overlying said second portion of said major surface, said first portion of said gate dielectric having a first thickness and said second portion of said gate dielectric having a second thickness substantially thinner than said first thickness; and a gate conductor overlying said first and second portions of said gate dielectric.
 10. The FET as claimed in claim 9, wherein said second portion of said gate dielectric has a step difference in thickness relative to said first portion of said gate dielectric.
 11. The FET as claimed in claim 10 wherein said semiconductor region is disposed in a semiconductor-on-insulator (“SOI”) layer of a semiconductor-on-insulator (“SOI”) substrate having a buried insulator layer and a bulk semiconductor region disposed below said SOI layer.
 12. The FET as claimed in claim 9, wherein said gate dielectric consists essentially of a material having a dielectric constant K greater than that of silicon dioxide.
 13. The FET as claimed in claim 12, wherein said gate dielectric consists essentially of at least one material selected from the group consisting of: hafnium oxide, hafnium silicates, zirconium oxide, perovskite materials, ferroelectric dielectric materials, zeolites, and lead zirconium titanate.
 14. A method of fabricating a field effect transistor (“FET”), comprising: providing a semiconductor region including a channel region having an exposed major surface, said channel region including a first portion and a second portion, said first and second portions each occupying a substantial proportion of a total area of said major surface; forming a gate dielectric having a first portion overlying said first portion of said major surface and having a second portion overlying said second portion of said major surface, said first portion of said gate dielectric having a first thickness and said second portion of said gate dielectric having a second thickness substantially less than said first thickness; forming a gate conductor overlying said first and second portions of said gate dielectric and said first and second portions of said major surface; and forming a drain region and a source region in said semiconductor region, said source region having a conduction path through said channel region to said drain region.
 15. The method of fabricating a FET as claimed in claim 14, wherein said FET is operable in response to a gate voltage applied between said gate conductor and said source region to determine whether said FET is operated in at least: a) an essentially nonconductive state when a magnitude of said gate voltage is less than a first threshold voltage of said FET such that a source-drain current between said source region and said drain region has a negligible value; b) a first conductive state when said magnitude of said gate voltage is greater than said first threshold voltage and less than a second threshold voltage of said FET such that said source-drain current has a significant operating value; and c) a third conductive state when said magnitude of said gate voltage is greater than said second threshold voltage such that said source-drain current has a value of at least about ten times higher than said significant operating value.
 16. The method as claimed in claim 15, wherein said semiconductor region includes silicon and said step of forming said first and second portions of said gate dielectric includes implanting an oxidation retardant into said second portion of said major surface and forming said gate dielectric by thermal oxidation of the silicon in said semiconductor region.
 17. The method as claimed in claim 16, wherein said step of forming said gate dielectric includes masking at least one of said first portion or said second portion of said major surface and forming at least one of said first portion or said second portion of said gate dielectric on said major surface, and subsequently forming another one of said first portion or said second portion of said gate dielectric.
 18. The method as claimed in claim 17, wherein said step of forming said at least one of said first portion or said second portion of said gate dielectric includes forming an oxide of a semiconductor material included in said semiconductor region by thermal oxidation.
 19. The method as claimed in claim 18, wherein said step of forming said at least one of said first portion or said second portion of said gate dielectric includes forming a nitride of a semiconductor material included in said semiconductor region by thermal nitridation.
 20. The method as claimed in claim 17, wherein said step of forming said at least one of said first portion or said second portion includes depositing said gate dielectric. 